1. Field of the Invention
This invention pertains generally to liquid injectors, and more particularly to an apparatus and method for ejecting liquid from a microdevice.
2. Description of the Background Art
Liquid droplet injectors are widely used for printing in inkjet printers. Liquid droplet injectors, however, can also be used in a multitude of other potential applications, such as fuel injection systems, cell sorting, drug delivery systems, direct print lithography, and micro jet propulsion systems, to name a few. Common to all these applications, a reliable and low-cost liquid droplet injector which can supply high quality droplets with high frequency and high spatial resolution, is highly desirable.
Only several devices have the ability to eject liquid droplets individually and with uniform droplet size. Among the liquid droplet injection systems presently known and used, injection by a thermally driven bubble has been most successful of such devices due to its simplicity and relatively low cost.
Thermally driven bubble systems, which are also known as bubble jet systems, suffer from cross talk and satellite droplets. The bubble jet system uses a current pulse to heat an electrode to boil liquid in a chamber. As the liquid boils, a bubble forms in the liquid and expands, functioning as a pump to eject a column of liquid from the chamber through an orifice, which forms into droplets. When the current pulse is terminated, the bubble collapses and liquid refills the chamber by capillary force. The performance of such a system can be measured by the ejection speed and direction, size of droplets, maximum ejection frequency, cross talk between adjacent chambers, overshoots and meniscus oscillation during liquid refilling, and the emergence of satellite droplets. During printing, satellite droplets degrade image sharpness, and in precise liquid control, they reduce the accuracy of flow estimation. Cross talk occurs when bubble jet injectors are placed in arrays with close pitch, and droplets eject from adjacent nozzles.
Most thermal bubble jet systems place a heater at the bottom of the chamber, which loses significant energy to the substrate material. Additionally, bonding is typically used to attach the nozzle plate to its heater plate, which limits nozzle spatial resolution due to the assembly tolerance required. Moreover, the bonding procedure may not be compatible with IC precess, which could be important if the integration of microinjector array with controlling circuit is desired to reduce wiring and to ensure compact packaging.
To solve cross talk and overshoot problems, it has typically been the practice to increase the channel length or adding chamber neck to increase fluid impedance between the chamber and reservoir. However, these practices slow the refilling of liquid into the chamber and greatly reduce the maximum injection frequency of the device.
The most troublesome problem with existing inkjet systems is satellite droplet because it causes image blurring. The satellite droplets that trail the main droplet hit the paper surface at slightly different locations than the main one as the printhead and paper are in relative motion. There is no known effective means or method to solve the satellite droplet problem that is readily available and economical.
Accordingly, there is a need for a liquid droplet injection system that minimizes cross talk without slowing down the liquid refilling rate, thereby maintaining a high frequency response while eliminating satellite droplets, all without adding complexity to the design and manufacturing. The present invention satisfies thess needs, as well as others, and generally overcomes the deficiencies found in the background art.